For Sorcery (Grimoire) casting in particular -
LEGEND Core Rulebook : Page 194
"Casting Critical Successes
If the Sorcery (Grimoire) roll is a critical success, the spell costs no Magic Points to cast,
irrespective of the amount of Manipulation applied to it."
"Casting Failures
If the casting test fails the spell does not take effect and costs the caster a single Magic Point."
"Casting Fumbles
If the Sorcery roll is fumbled the spell fails and the sorcerer loses the full Magic Point cost of
the spell."
And as folks have stated above The Common Magic skill description only states the following.. (p.48)
"If a Common Magic roll is a critical success, then the spell costs 1 less Magic Point (to a minimum of 1) to cast."
"If the roll fumbles, then the Adventurer expends all the Magic Points for the spell’s Magnitude
but the spell still fails."
Divine, of course, uses no Magic Points in casting.
I'd love someone from Mongoose to clarify if this as stated above is the full interpretation, or whether a line of text was omitted (possibly) in the Common Magic description? I was a little surprised that Common Magic failure does not expend 1 MP is my overall feeling on the matter. If taken verbatim, the above interpretation is to the letter.
I actually like the house ruling described further above:
"I've always assumed that on an ordinary success, you spend the magic points and get the result that you want. On a failure, you spend the magic points but don't get the result that you want. And on a fumble, you spend the magic points and something BAD happens."
That I may have to steal. It's a cleaner and more logical way to handle it for both Common and Sorcery.